1. Field of the Invention
The present invention relates to a laser apparatus as being a driver laser for an extreme ultraviolet light source apparatus used for an exposure apparatus and an extreme ultraviolet light source apparatus using such laser apparatus.
2. Description of the Related Art
In recent years, along with a progress in a miniaturization of semiconductor device, miniaturization of transcription pattern used in photolithography in semiconductor process has developed rapidly. In the next generation, microfabrication to the extent of 70 nm to 45 nm, or even to the extent of 32 nm and beyond will be required. Therefore, in order to comply with the demand of microfabrication to the extent of 32 nm and beyond, development of such on exposure apparatus combining an extreme ultraviolet (EUV) light source for a wavelength about 13 nm and a reflection-type reduction projection optical system is expected.
As the EUV light source, there are three possible types, which are a laser produced plasma (LPP) light source using plasma generated by irradiating a target with a laser beam, a discharge produced plasma (DPP) light source using plasma generated by electrical discharge, and a synchrotron radiation (SR) light source using orbital radiant light. Among these light sources, the LPP light source has the advantage of obtaining extremely high optical intensity close to the black-body radiation because plasma density can be made higher than the DPP light source and the SR light source. Moreover, the LPP light source has the advantage of obtaining a strong light with a desired wavelength band by selecting a target material. Furthermore, the LPP light source is a point light source which has no electrode located around a luminous point and has a nearly isotropic angular distributions. Therefore, extremely wide collecting solid angle can be acquired. The LPP light source with the above-mentioned advantages has attracted attention as a light source for EUV lithography which requires more than several dozen to several hundred watt power.
In the EUV light source apparatus with the LPP system, firstly, a target material supplied inside a vacuum chamber is irradiated with a laser light to be ionized and thus generate plasma. Then, a cocktail light with various wavelength components including an EUV light is emitted from the generated plasma. The EUV light source apparatus collects the EUV light by reflecting the EUV light using an EUV collector mirror which selectively reflects the EUV light with a desired wavelength component, such as a 13.5 nm wavelength component, for instance. The collected EUV light enters an exposure apparatus. On a reflective surface of the EUV collector mirror, a multilayer coating, with a structure in that thin coatings of molybdenum (Mo) and thin coatings of silicon (Si) are alternately stacked, for instance, is formed. The multilayer coating has a high reflectance ratio (of about 60% to 70%) for the EUV light with a 13.5 nm wavelength.
US patent application Laid-Open No. 2007/1131 (hereinafter to be referred to as patent reference 1) discloses a driver laser for an EUV light source apparatus with the LPP system. Specifically, this driver laser is in a MOPA (master oscillator and power amplifier) system using a CO2 gas laser. The driver laser with the MOPA system amplifies a light from a master oscillator (MO), which outputs a pulse laser light, using three power amplifiers.
Moreover, Japanese patent application Laid-Open No. 2006-135298 (hereinafter to be referred to as patent reference 2) discloses a driver laser for an EUV light source apparatus with the LPP system. This driver laser uses a CO2 gas laser as an MO. In a CO2 gas laser which normally oscillates, oscillating lines will be concentrated in P(20). On the other hand, the driver laser disclosed in the patent reference 2 prevents the oscillating lines from being concentrated in the P(20) by inserting a wavelength selecting element into a resonator of the CO2 gas laser. Moreover, patent reference 2 discloses a driver laser having a structure in that a solid laser and two nonlinear crystals transforming a light from the solid laser into a light with a broad wavelength distribution of around 10.6 μm are used as an MO. In this driver laser, a light from the MO is amplified by an amplifier using CO2 gas as an amplifying agency.
Furthermore, US patent application Laid-Open No. 2007/30865 (hereinafter to be referred to as patent reference 3) discloses a middle-infrared ray (MIR) laser for mainly measuring concentration of gas in the atmosphere with high accuracy. This MIR laser uses a quantum cascade laser as a wavelength tunable semiconductor laser for middle-infrared band.
However, it is a problem that the use of the CO2 gas laser being a high power molecular laser as an MO for the same type of the CO2 gas laser will make the structure of a controller such as a pulse controller complicated, and moreover, will make handling of the controller more difficult.
On the other hand, in a case where a laser light from the solid laser is transformed using the nonlinear crystal, or the like, and the transformed laser light is amplified using the CO2 gas laser, in order to improve the amplification efficiency, it is required that a wavelength of the light outputted from the nonlinear crystal is adjusted to an amplifiable line of the CO2 gas laser. However, a light with a wavelength matching up with an amplifiable line of a molecular gas laser such as the CO2 gas laser is difficult to generate by a disparate laser apparatus such as a solid laser combined with a nonlinear crystal.